In the manufacture of paper products, such as facial tissue, bath tissue, paper towels, dinner napkins and the like, a wide variety of product properties are imparted to the final product through the use of chemical additives applied in the wet end of the tissue making process. Two of the most important attributes imparted to tissue through the use of wet end chemical additives are strength and softness. Specifically for softness, a chemical debonding agent is normally used. Such debonding agents are typically quaternary ammonium compounds containing long chain alkyl groups. The cationic quaternary ammonium entity allows for the material to be retained on the cellulose via ionic bonding to anionic groups on the cellulose fibers. The long chain alkyl groups provide softness to the tissue sheet by disrupting fiber-to-fiber hydrogen bonds in the sheet. The use of such debonding agents is broadly taught in the art. Such disruption of fiber-to-fiber bonds provides a two-fold purpose in increasing the softness of the tissue. First, the reduction in hydrogen bonding produces a reduction in tensile strength thereby reducing the stiffness of the sheet. Secondly, the debonded fibers provide a surface nap to the tissue web enhancing the xe2x80x9cfuzzinessxe2x80x9d of the tissue sheet. This sheet fuzziness may also be created through use of creping as well, where sufficient interfiber bonds are broken at the outer tissue surface to provide a plethora of free fiber ends on the tissue surface. Both debonding and creping increase levels of lint and slough in the product. Indeed, while softness increases, it is at the expense of an increase in lint and slough in the tissue relative to an untreated control. It can also be shown that in a blended (non-layered) sheet that the level of lint and slough is inversely proportional to the tensile strength of the sheet. Lint and slough can generally be defined as the tendency of the fibers in the paper web to be rubbed from the web when handled.
It is also broadly known in the art to use a multi-layered tissue structure to enhance the softness of the tissue sheet. In this embodiment, a thin layer of strong softwood fibers is used in the center layer to provide the necessary tensile strength for the product. The outer layers of such structures are composed of the shorter hardwood fibers, which may or may not contain a chemical debonder. A disadvantage to using layered structures is that while softness is increased the mechanism for such increase is believed due to an increase in the surface nap of the debonded, shorter fibers. As a consequence, such structures, while showing enhanced softness, do so with a trade-off in the level of lint and slough.
It is also broadly known in the art to concurrently add a chemical strength agent in the wet-end to counteract the negative effects of the debonding agents. In a blended sheet, the addition of such agents reduces lint and slough levels. However, such reduction is done at the expense of surface feel and overall softness and becomes primarily a function of sheet tensile strength. In a layered sheet, strength chemicals are added preferentially to the center layer. While this perhaps helps to give a sheet with an improved surface feel at a given tensile strength, such structures actually exhibit higher slough and lint at a given tensile strength, with the level of debonder in the outer layer being directly proportional to the increase in lint and slough.
There are additional disadvantages with using separate strength and softness chemical additives. Particularly relevant to lint and slough generation is the manner in which the softness additives distribute themselves upon the fibers. Bleached Kraft fibers typically contain only about 2-3 milli-equivalents of anionic carboxyl groups per 100 grams of fiber. When the cationic debonder is added to the fibers, even in a perfectly mixed system where the debonder will distribute in a true normal distribution, some portion of the fibers will be completely debonded. These fibers have very little affinity for other fibers in the web and therefore are easily lost from the surface when the web is subjected to an abrading force.
Therefore there is a need for a means of reducing lint and slough in soft tissues while maintaining softness and strength.
It has now been discovered that the amount of lint and slough can be reduced for a given tensile strength or level of debonder chemical. This is accomplished by incorporating into the paper sheet a synthetic polymer having a portion of its structure derived from the polymerization of acrylamide and thereby containing pendant amide groups capable of increasing interfiber bonding. The synthetic polymer also contains an aliphatic hydrocarbon moiety. While not wishing to be bound by theory, it is believed that the synthetic polymer eliminates the potential for formation of totally debonded fibers. The aliphatic hydrocarbon portion of the molecule enables a significant level of debonding to occur and insures that the product has good surface nap or xe2x80x9cfuzzyxe2x80x9d feel. Yet, these fibers retain a significant bonding potential due to the presence of the pendant bonding functionality and as such the fibers remain anchored to the web. As such, fibers treated with these synthetic polymers produce a tissue web having lower lint and slough at a given tensile strength than a web prepared with conventional softening agents or a combination of conventional softening agents and conventional strength agents.
Hence, in one aspect, the invention resides in a soft paper sheet, such as a tissue sheet, comprising a synthetic polymer having hydrogen bonding capability and containing a hydrophobic aliphatic hydrocarbon moiety, said polymer having the following structure: 
where:
w, x, y, zxe2x89xa71;
vxe2x89xa70;
R0, R0xe2x80x2, R0xe2x80x3, R1, R2, R2xe2x80x2, R2xe2x80x3 are independently H, C1-4 alkyl;
R3=a C4 or higher linear or branched, saturated or unsaturated, substituted or unsubstituted hydrophobic aliphatic hydrocarbon moiety;
Z1=a bridging radical whose purpose is to attach the R3 moiety to the polymer backbone. Suitable Z1 radicals include but are not limited to xe2x80x94COOxe2x80x94, xe2x80x94CONHxe2x80x94, xe2x80x94Sxe2x80x94, xe2x80x94OCOxe2x80x94, xe2x80x94NHCOxe2x80x94, xe2x80x94Oxe2x80x94, aryl, xe2x80x94CH2xe2x80x94;
F=a salt of an ammonium cation. The purpose of the F group is to provide a cationic charge to the polymer. Alternatively F may contain a tertiary amine group capable of being protonated, such that in an acidic environment, the group will possess a cationic charge and thereby be capable of being retained on the cellulose.
R4=an aldehyde functional hydrocarbyl radical, including but not limited to xe2x80x94CHOHCHO or xe2x80x94CHOHCH2CH2CHO.
Diallyldimethylammonium chloride can be used for incorporating the cationic monomer into the synthetic polymer. When Diallyldimethylammonium chloride is used the synthetic polymer has the following structure: 
where
R0, R0xe2x80x2, R0xe2x80x3, R1, R3 R4, Z1, v, w, x, y, z are as defined above.
In another aspect, the invention resides in a method of making a soft, low lint paper sheet, such as a tissue sheet, comprising the steps of: (a) forming an aqueous suspension of papermaking fibers; (b) depositing the aqueous suspension of papermaking fibers onto a forming fabric to form a web; and (c) dewatering and drying the web to form a paper sheet, wherein a synthetic polymeric additive is added to the aqueous suspension of fibers or to the web, said polymeric additive having the following structure: 
where:
w, x, y, zxe2x89xa71;
vxe2x89xa70;
R0, R0xe2x80x2, R0xe2x80x3, R1, R2, R2xe2x80x2, R2xe2x80x3 are independently H, C1-4 alkyl;
R3=a C4 or higher linear or branched, saturated or unsaturated, substituted or unsubstituted aliphatic hydrocarbon moiety;
Z1=a bridging radical whose purpose is to attach the R3 moiety to the polymer backbone. Suitable Z1 radicals include but are not limited to xe2x80x94COOxe2x80x94, xe2x80x94CONHxe2x80x94, xe2x80x94Sxe2x80x94, xe2x80x94OCOxe2x80x94, xe2x80x94NHCOxe2x80x94, xe2x80x94Oxe2x80x94, aryl;
F=a salt of an ammonium cation. The purpose of the F group is to provide a cationic charge to the polymer. Alternatively F may contain a tertiary amine group capable of being protonated, such that in an acidic environment, said group will possess a cationic charge and thereby be capable of being retained on the cellulose; and
R4=an aldehyde functional hydrocarbyl radical, including but not limited to xe2x80x94CHOHCHO or CHOHCH2CH2CHO.
Diallyldimethylammonium chloride can be used to incorporate the cationic monomer into the synthetic polymer. When Diallyldimethylammonium chloride is used, the synthetic polymer has the following structure: 
where
R0, R0xe2x80x2, R0xe2x80x3, R1, R3 R4, Z1, v, w, x, y, z are as defined above.
As used herein, xe2x80x9caliphatic hydrocarbon moietiesxe2x80x9d are functional groups derived from a broad group of organic compounds, including alkanes, alkenes, alkynes and cyclic aliphatic classifications. The aliphatic hydrocarbon moieties can be linear or branched, saturated or unsaturated, substituted or non-substituted.
The synthetic polymers as described herein may be water soluble, organic soluble or soluble in mixtures of water and water miscible organic compounds. Preferably they are water-soluble or water dispersible but this is not a necessity of the invention.
The amount of the synthetic polymeric additive added to the papermaking fibers or the paper or tissue web can be from about 0.02 to about 4 weight percent, on a dry fiber basis, more specifically from about 0.05 to about 3 weight percent, and still more specifically from about 0.1 to about 2 weight percent. The synthetic polymer can be added to the fibers or web at any point in the process, but it can be particularly advantageous to add the synthetic polymer to the fibers while the fibers are suspended in water. This can include, for example, addition in the pulp mill or to the pulper, a machine chest, the headbox or to the web prior to being dried where the consistency is less than about 80 percent.